The present invention relates to acoustical doors, and more particularly, to an acoustical door system having improved perimeter seals.
Sound-retardant doors and windows are used extensively not only in performing arts centers, concert halls, broadcast studios, auditoriums and movie theaters, but also in critical industrial, aerospace and defense installations, as well as in other locations where noise control and/or voice privacy may be required. When sound energy within a room meets a barrier, part of its energy can be absorbed by the barrier and part reflected. The remaining vibrating energy puts the barrier into motion and it becomes a second transmitter, thereby radiating sound into adjacent areas. The construction materials and techniques used in enclosing the source noise will govern the amount of energy transferred from the source area to adjoining spaces.
Sound energy impinging on barriers can be absorbed with embedded porous materials in which the sound waves produce motion, thereby doing work and dissipating energy as heat. To reduce the amount of energy radiated by a barrier, a damping material may be applied to decrease the overall motion of the barrier. Typically, such materials are limp, and provide excellent vibration control and damping. The effectiveness of a barrier material thus is enhanced by an optimum combination of absorptive and damping materials. In this respect, although a limp-mass material offers good sound barrier properties, it is not practical for exterior applications. Most sound barrier exterior materials are hard, stiff, dense and very reflective. The composite of outer stiff and inner limp material functions as an effective sound barrier. Such composites are used in sound-retardant doors, also known as acoustical doors.
In the past, the common practice in sound-retardant door designs generally followed that for commercial freezer and refrigerator doors, using wood, metal, cork, mineral insulation, lead sheets and other materials in a thick sandwich configuration. In general, each material, according to its mass, retards penetration of a segment of the particular sound frequencies involved. However, doors were massive--4, 5 and 6 inches thick--and required special hardware derived from commercial refrigeration doors. The thick doors, although relatively good sound barriers were aesthetically unappealing and presented problems with fire safety, ease of entrance and exit, and compatibility with the building locking system.
More recently, the engineering of sound barriers has developed to the point where fairly effective acoustical doors of 13/4 inch thickness are available. Examples of the designs of prior art acoustical doors are found in U.S. Pat. Nos. 3,273,297, 3,295,273 and 3,319,738.
The performance of a sound barrier is currently given in terms of a "sound transmission class" (STC). STC is a single number rating derived from measured values of sound Transmission Loss (TL) in accordance with American Society for Testing and Materials (ASTM) standards. TL through a door is a measure of its effectiveness in preventing the sound power incident on one side from being transmitted through it and radiated on the other side, taking into account the area of the door and the absorption in the receiving room. The STC provides a single number estimate of a door or a window's performance for certain common sound reduction applications.
To provide a tangible measure of STC values, sound-attenuation classifications and acoustic isolation criteria (voice range only) for construction of various sound-sensitive rooms have been defined. The criteria are as follows:
______________________________________ Sound Group I 30 .ltoreq. STC &lt; 40 Sound Group II 40 .ltoreq. STC &lt; 45 Sound Group III 45 .ltoreq. STC &lt; 50 Sound Group IV 50 .ltoreq. STC ______________________________________
Sound levels in secure facilities which serve as private offices or laboratories comprise Sound Group II. Sound Group III rooms are described as standard executive suites, open work spaces, briefing or conference rooms, planning and training rooms, projector rooms, and auditoriums that do not require sound amplification. Auditoriums with sound reinforcements, combat centers, war rooms and battle management areas all fall within Sound Group IV.
In the laboratory, TL measurements are conducted in two adjacent highly reverberant rooms, presenting a diffused sound field, requiring walls with acoustical properties far superior to the test specimen. The specimen to be tested is sealed in an opening between the two rooms and a calibrated noise source and frequency spectrum is activated. The same rotating microphone is used in each room to transmit measured sound levels to analyzers that determine the TL in decibels (dB) at each of 18 one-third octave bands between 100 and 5,000 Hz. The middle 16 TL readings between the 125 and 4,000 Hz one-third octave bands are plotted against a standard contour curve as established by ASTM standards. The result is a convenient single number rating (STC) that covers the primary speech frequencies and is an easy way for users to rank the relative effectiveness of sound barrier products.
Outside the controlled laboratory environment, however, factors affecting the acoustical performance of a specific door assembly also include the quality of perimeter seals, hardware, frame, and integration of the frame into the surrounding wall. A general rule of thumb is that if air, light or water can pass through gaps around a barrier, so can sound, and the effectiveness of well-designed acoustical doors can be destroyed by even relatively small peripheral openings. Thus, there is some discrepancy between an STC rating obtained in the laboratory and the actual effectiveness of the sound barrier when installed on site.
An operating test may be performed in the lab which measures the sound retardant effectiveness of the door and surrounding frame and seals. Such a test provides a reasonably accurate determination of the STC of the assembled acoustical door system.
A field test is a commonly used term referrinq to a test conducted at the job site, by a qualified acoustical consultant, to verify the operating test results for a particular barrier. The test provides a noise isolation class (NIC), a single number rating derived from measured values of noise reduction through the item tested, in accordance with ASTM standards. These figures are used to provide a Field Sound Transmission Class (FSTC). There is typically a difference between FSTC and operating STC ratings, but this difference should be minimal, no more than five points in most cases.
Currently, some acoustical doors make use of camming hinges, which lift the door when opened. Thus, the gap at the bottom of the door between the door and the door step is closed when the door is shut. Disadvantageously, current mortised seals disposed at the bottom of the door are attached to only one side, and interfere with a peripheral seal at that lower location. Various other designs for seals or gaskets on the periphery of the doors have been developed, in particular, as shown in U.S. Pat. No. 3,221,376. However, prior designs contact one panel of the door and require a large amount of compressive force to ensure a tight acoustic seal. Such large compression results in the door "bouncing back" upon unlatching.
As opposed to single doors, pairs of doors present a greater perimeter footage to be sealed around the periphery, as well as the central gap between the two doors. Presently, there is no specific standards for testing sound reduction through double doors. Based on empirical testing, it is not unusual to experience drops in laboratory STC values of up to 15 classes for the same doors installed on site as a pair. Typically, a sealing strip known as an astragal covers the central gap between the two doors.
The present invention is directed to the need for an effective acoustical door for single and double doors which is relatively simple and inexpensive, yet performs better than acoustical doors of the prior art.